The present invention relates to abrasive grains, especially diamond grains, having thick, sinterable or sintered coatings thereon.
The present invention also relates to cutting tools such as wire saws and saw blades of the kind used for sawing hard and/or abrasive materials such as, for example, granite, marble, concrete, asphalt, and the like which are prepared from the thick, sinterable abrasive particles of the present invention.
U.S. Pat. No. 3,316,073 to Kelso, incorporated herein by reference, describes spherical abrasive bodies comprising a central abrasive particle surrounded by a matrix forming material. According to the '073 patent, the matrix material contains a sinterable material which upon application of heat will weld together to form a permanent strong bond for the abrasive particles. In some cases a temporary binder may be employed to make the pellets self-sustaining and strong enough to withstand handling prior to formation of the finished fired product. The bond volume to abrasive volume ratio is said to be at least 9 to 1. Such abrasive bodies may be sintered to form a finished tool.
U.S. Pat. No. 4,770,907 to Kimura, incorporated herein by reference, discloses a method of producing metal coated abrasive grains suitable for the manufacture of metal bonded diamond tools by the steps of initially preparing a slurry of metal powder with a solution of an agglomerating binder dissolved in an organic solvent, agitating the slurry to homogeneously suspend the metal powder therein, and spraying the slurry using an atomizing nozzle into a fluidized suspension of abrasive grains within a fluid bed granulator to progressively build and dry a uniform spherical coating of the slurry on each individual abrasive grain.
However, neither Kelso nor Kimura disclose or suggest the use of metal pellets devoid of abrasive grains in order to control the abrasive granule concentration in the finished tool. In the absence of such metal pellets devoid of abrasive grains; i.e., "dummy pellets," it would be necessary to provide a wide variety of coating thicknesses to be capable of varying the concentration of abrasive particles in the finished tool. Furthermore, Kelso neither discloses nor suggests sintering the coated abrasive grains prior to manufacture of the finished tool.
Conventionally, the cutting of hard materials such as, for example, granite, marble, filled concrete, asphalt and the like, has been achieved using rotary or circular diamond saws. The blade of saws of this type comprises a circular steel disk having a plurality of spaced segments about its cutting edge, which segments consist essentially of diamond abrasive bonded in a suitable alloy or metal matrix, such as bronze or cobalt, for example. The diamond abrasive typically is either a single crystal natural diamond or a single crystal manufactured diamond. U.S. Pat. No. 4,883,500 proposes the use of a combination of thermally stable polycrystalline diamond cutting elements and single crystal cutting elements dispersed in a bonding matrix.
A variety of problems have plagued the artisan in fabricating segments and saw blades containing such segments. For example, to optimize diamond saw blade performance, it would be desirable to prevent agglomeration of diamond in the metal bond powders during mixing and blending. This would permit maximum utilization of the diamond abrasive crystals. Further, it would be convenient to have controlled porosity within each segment to provide localized coolant during use, to minimize thermal degradation of diamond, and/or the bond metal.
In granite sawing with rotary saw blades, for example, lateral deflection of the blade as it enters harder areas of the stone can give rise to non-flat portions of the surface being cut. This leads to the need for expensive grinding and reduces the profit for the stone processor. One solution to this problem in the field has been to make sandwich segments with differential wear patterns to provide a matching track on the stone which reduces the chances of lateral blade deflection during the cut. Until now, the differential wear has been achieved by either varying concentration of the diamond across the face, or by adding wear resistant material in the outer portions of the segments, such as proposed in U.S. Pat. No. 4,883,500, cited above. These methods use the technique for filling the pressing cavity in three consecutive cold pressing steps, and then hot pressing to sinter the metal matrix powder in a separate operation.
It has been common practice for several decades to blend or mix diamond abrasive powders with fine metal powders to form segments by hot pressing in a sintering press. Air-borne fine metal powders, if toxic or carcinogenic, represent a potential health hazaed. In addition, differences in size, shape, and density between diamond and metal make production of homogeneous mixtures difficult. Also, it would be convenient to be able to automate mold loading.